1. Field of the Invention
This invention relates to a method of and an apparatus for forming images such as half-tone images or colored images, and more particularly to an image forming method and apparatus for forming images by using a number of micro-area dots (or pattern elementary marks).
The term "dots" (pattern elementary marks) used herein refers to what accords with forming marks of a minimum unit forming an image pattern such as a picture, a character, a figure or a graph, for example, recorded dots on a recording medium in dot recording. Incidentally, in image formation, it is to be understood that where one picture element is formed by one pattern elementary mark, the pattern elementary mark and the picture element accord with each other, but where one picture element is formed by a plurality of pattern elementary marks, the pattern elementary marks and the picture element are different.
2. Description of the Prior Art
For example, in the ink jet type dot recording wherein colored ink droplets are discharged to form ink dots on a recording medium (such as paper, plastics or ceramics) to thereby effect recording and formation of patterns such as images, it has already been proposed to effect recording with the size (for example, dot diameter) of colored ink dots on the recording medium varied for the reproduction of density gradients or tones. Particularly, in an ink jet recording system using a piezo element, it is relatively easy to take the correspondence between a voltage applied to the piezo element and the size of the diameter of the ink dots on the recording medium controlled by the applied voltage and accordingly, the control of gradient levels can be easily accomplished by controlling the dot diameter through a relatively simple control circuit.
Now, when control of the gradient level of a pattern is made by controlling the diameter of the ink dots, namely, the size of the pattern elementary marks, examination of the variation in gradient level of the pattern (for example, the variation in average optical reflection density of the formed pattern portion) relative to the variation in size of the pattern elementary marks reveals that there exists an area in which a relatively great variation in gradient level occurs relative to the variation in size of the pattern elementary marks and an area in which the gradient level is not so much varied.
This will now be analyzed with respect to the relation between the optical reflection density Dd of a dot and the number of dots arranged per 1 mm in one-dimensional direction (hereinafter referred to as the PEL number) in a case where a colored dot having an optical reflection density Dd (reflection factor Ad=10.sup.-Dd) is used on a material (a recording medium such as paper) having an optical reflection density D.sub.0 (reflection factor A.sub.0 =10.sup.-D0) and the diameter d of the dot is varied to thereby vary the average optical reflection density D (reflection factor A=10.sup.-D) per unit area (1 mm square) of the pattern portion formed by the dot. First, assuming that dots are uniformly arranged at a pitch T(=1/n) on the above-mentioned material, then the reflection factor A of the pattern portion per unit area is: ##EQU1##
Accordingly, the average optical reflection density D in this case may be expressed as follows. In the following, "log" indicates common logarithms whose base is 10. ##EQU2## For example, where a pattern is formed on a material (such as white paper) having an optical reflection density D.sub.0 =0.1 by a colored dot having an optical reflection density Dd=1.0, the average optical reflection density D of the pattern portion per unit area is given as follows by equation (2) above, and this is graphically shown by curve I in FIG. 1 of the accompanying drawings. ##EQU3##
In FIG. 1, the abscissa represents the dot duty ratio d/T in one-dimensional direction and the ordinate represents the average optical reflection density D. Also, in FIG. 1, curves II and III show the cases where the dot optical reflection density is 1.4 and 0.6, respectively. If the diameter d of dot becomes greater and exceeds a pitch T, that is, if the duty ratio d/T exceeds 1.0, adjacent dots overlap each other and therefore, the average optical reflection density D begins to be gradually saturated relative to the variation in the dot diameter d and in calculation, becomes completely saturated at d=.sqroot.2T. Incidentally, as regards the curves I, II and III of FIG. 1, the optical reflection density Dd=1.6, 1.4 and 0.6 respectively correspond to colored inks containing 2.0%, 4.5% and 0.5% (all by weight) of dye or pigment, respectively.
FIG. 1 shows the manner of variation in the average optical reflection density D of the formed pattern relative to the variation in the duty ratio d/T of dot with respect to colored dots having a constant dot pitch T and different optical reflection densities Dd, and if equation (2) above is used to show the manner of variation in the average optical reflection density D of the formed pattern relative to the variation in the duty ratio d/T of dot with respect to colored dots having a constant optical reflection density Dd and different dot pitches T, it will be as shown in FIG. 2 of the accompanying drawings. In FIG. 2, curves I, II and III show the manner of variation in the average optical reflection density D of the pattern relative to the variation in dot diameter d in the cases where the optical reflection density Dd of dot is 1.0 and the dot pitches T (and PEL numbers) are 200 .mu.m (5 PEL), 500/3 .mu.m (6 PEL) and 250 .mu.m (4 PEL), respectively.
The curves in FIGS. 1 and 2 were all obtained from the values calculated by equation (2) above, but as an actual measuring method, the optical reflection density of the pattern forming material (for example, the above-mentioned ink) is obtained as by uniformly applying the pattern forming material to an area of 10 mm square and measuring it by a commercially available densitometer, and the average optical reflection density of the pattern portion is obtained as by measuring the pattern portion formed by pattern elementary marks arranged in an area of 10 mm square, by the use of a densitometer. Actually, in either case, a reference value of measurement may be determined in advance by the use of standard white paper or the like having a reflection density of about 0.1.
Now, as will be appreciated from the various curves shown in FIGS. 1 and 2, when an attempt is made to obtain a variation in gradient level (average optical reflection density) of the formed pattern by varying the dot diameter, i.e., the pattern elementary mark size, the variation in gradient level of the pattern relative to the variation in size of the pattern elementary marks is not uniform and particularly, in an area wherein the size of the pattern elementary marks is small and an area wherein the size of the pattern elementary marks is large, the gradient level of the pattern is not so much varied relative to the variation in size of the pattern elementary marks. As will be understood from FIGS. 1 and 2, this is a phenomenon equally noted irrespective of the optical reflection density (coloring density) of the pattern elementary marks themselves and irrespective of the arrangement pitch of the pattern elementary marks. Thus, the area in which the variation in gradient level of the pattern is small relative to the variation in size of the pattern elementary marks is an area very inefficient to obtain a predetermined variation in gradient level.
With regard to the reproduction of density gradient, the following methods have heretofore been proposed.
A first method is to control the amount of liquid discharged from an ink jet head to thereby vary the diameter of dots printed and express gradient.
A second method is to construct a picture element by a matrix comprising, for example, 4.times.4 micro picture elements without changing the dot diameter and use the dither method for this matrix to reproduce density gradients. With the first method, however, it is difficult to secure a great range from the minimum dot diameter to the maximum dot diameter and reproduction of only several tones is possible. Accordingly, this method has been unsatisfactory for printing-out of television images or photographic images.
The second method overcomes the disadvantage of the first method and with this method, it is possible to reproduce seventeen tones of gradient, for example, in case a picture element is constructed by 4.times.4 matrix. In this method, however, as compared with the first method, the printing speed is reduced by 1/16 because a picture element is constructed by 16 elementary marks or it is necessary to achieve a higher printing speed by increasing the number of printing heads 16 times. This not only has rendered the construction of the printing head complex, but also has led to increased sizes in the electric circuits for image processing by the dither method and accordingly to a greatly increased cost for such apparatus.